This invention relates generally to sightglass assemblies and, more particularly, to a sightglass assembly for use in connection with a laser flowmeter.
The advent in recent years of laser Doppler flowmeters for studying fluid flow dynamics has created a need for sightglass assemblies having certain performance characteristics required for use with laser flowmeters. In a laser Doppler flowmeter, two laser light beams are passed through a fluid medium at a slight angle with respect to one another such that the beams intersect at a point within the fluid medium. Upon passing through the fluid medium, interference effects between the two intersecting coherent laser beams are detected and measured by a receiving photodetector optics system.
The determination of the fluid flow rate from the interference effects between the laser beams is based upon the principle of the Doppler shift. Particles within the fluid scatter the laser light and cause a small shift in its frequency as observed from the receiving optics system. This shift in frequency is generally proportional to the velocity of the particles and thus can be measured and used to determine the velocity of particles in the fluid. From the particle velocity, the fluid velocity may be inferred. In practice, it is found that most liquids, for example, conventional hydraulic fluids, have sufficient concentrations of naturally occurring fine-grained particulate matter for this purpose. A small amount of particulate matter may be seeded into fluids which do not contain a sufficient naturally occurring concentration of particulate matter.
Laser flowmeters offer significant advantages over other types of flowmeters. Most importantly, laser flowmeters can detect more rapid changes in flow speed than can be detected with conventional flowmeter devices. Fluctuations in flow velocities can be detected at frequencies up to 50,000 cycles per second with a laser flowmeter. Also, the laser flowmeter does not require a probe or other device to be inserted into the flow stream, so problems due to turbulence generated by conventional flowmeter probes inserted in the flow stream are eliminated. Furthermore, laser flowmeters are more accurate than conventional flowmeters both at very high and very low flow velocities.
The use of a laser flowmeter requires a pair of optically transparent windows on opposites sides of the fluid medium to be studied. Each window will necessarily have its inner surface in contact with the fluid medium. The outer surface of the two windows will normally be flat and parallel to one another to avoid unwanted refraction effects. When fluids at elevated pressures are to be studied, high pressure sightglasses must be employed.
Where high pressure fluid flow within a tube is to be studied, it is necessary that the interior surface of each sightglass be flush with the interior wall surface of the tube. This is to avoid problems due to turbulence generated in the fluid as it passes the sightglass by an irregular juncture between the interior wall of the tube and the interior surface of the sightglass. At the same time, it is also a particular requirement of a laser flowmeter that the interior surfaces of the sightglasses be highly polished and free of scratches. In practice, it has heretofore been found quite difficult, if not impossible, to meet both of these requirements. Traditional single-walled sightglasses designed to withstand high fluid pressures have been typically constructed of thickwalled glass having a conically beveled edge. Such a traditional sightglass is seated in a wall port having a matching conically beveled inside edge so as to obtain a self-sealing effect from the pressurized fluid behind the wide portion of the sightglass. An elastomeric sealant is normally applied between the edges of the sightglass and the wall port to prevent fluid leakage and to prevent uneven stresses from developing within the sightglass. In practice, it has been found extremely difficult to construct a sightglass having a highly polished interior surface which, upon being installed and seated with a sealant in a matching wall port, provides an essentially flush juncture between the interior surface of the sightglass and the adjacent interior wall surface. To obtain a flush juncture it has normally been found necessary to grind the interior surface of the sightglass flush with the interior wall surface after the sightglass has been installed. This, however, inevitably introduces scratches in the sightglass surface due to metal particles abraded from the wall surface. When the glass surface is scratched, a portion of the laser beam is scattered by the sightglass surface and performance of the laser flowmeter is thereby impaired.
Accordingly, it is a general object of the present invention to provide a sightglass assembly for observation of high pressure fluid in a hydraulic line or other flow tube.
It is a further object of the present invention to provide a high pressure sightglass assembly for use in connection with a laser flowmeter.
It is also an object of the present invention to provide a prepolished high pressure sightglass assembly which may be installed with its interior surface being flush with the interior surface of a fluid flow tube without any necessity of post-installation polishing.
It is another object of the present invention to provide a high pressure sightglass assembly which can be installed in a fluid flow tube and which does not generate tubulence in fluid flowing past it.